In an optical communication network, optical signals having a plurality of optical channels at individual wavelengths, called “wavelength channels”, are transmitted from one location to another, typically through a length of optical fiber. An optical cross-connect module allows switching of optical signals from one optical fiber to another. A wavelength-selective optical cross-connect module, or a reconfigurable optical add-drop module (ROADM), allows wavelength-dependent switching, that is, it allows certain wavelength channels to be switched from a first optical fiber to a second optical fiber while letting the other wavelength channels propagate in the first optical fiber, or it allows certain wavelength channels to be switched to a third optical fiber. The new generation of ROADMs, which can switch any input wavelength to any output port, are often referred to as wavelength selective switches (WSSs).
Of the WSS architectures presently available, an architecture based on free-space optics, and including a switching engine such as a micro-electro-mechanical system (MEMS) array or a liquid crystal (LC) array, is one of the most versatile and high-performance architectures. For example, U.S. Pat. No. 6,707,959 to Ducellier et al. and U.S. Pat. No. 7,162,115 to Brophy et al, which are incorporated herein by reference, both disclose high performance optical switches.
Since WSSs are generally deployed at various nodes of an optical network, they must perform reliably in harsh environments characterized by a wide range of temperature and humidity. Accordingly, each WSS is typically packaged using a hermetic enclosure. Unfortunately, due to the relatively large footprint of the free-space optics, and due to the large number of electrical connections to the optical switching engine, mechanical packaging of WSSs represents a considerable technical challenge.
One approach to providing the required electrical connections within a hermetic enclosure has been to use a flexible printed circuit board (flex-PCB). The flex-PCB provides the electrical connection between the switching engine and the control system installed outside the enclosure, while mechanically de-coupling the switching engine from an internal multi-pin hermetic electrical connector mounted on a wall of the enclosure. Unfortunately, it takes a long time and considerable operator skill and effort to fit all the fiber feed-through, flex-PCBs, and to fit and align the optical elements inside the package, which increases the cost of the assembly and reduces manufacturing yields. Furthermore, a WSS enclosure built using this technology has a relatively large footprint since the optics, the flex-PCB, and the multi-pin hermetic connector need to be accommodated inside the package. The large size of prior art WSS enclosures is considered a drawback because telecom system providers are strongly motivated to increase the element density of their circuit cards, to facilitate a decrease in the system size and cost.